Magnetic resonance (MR) imagers are used to obtain images of objects by a process known as magnetic resonance imaging (MRI). Images of an object typically include a plurality of two-dimensional (2D) image sections (slices). Each slice includes a plurality of pixels, at different locations within the slice.
The acquisition of magnetic resonance images from an object, e.g., a heart, brain, or breast, generally includes subjecting the object to a static magnetic field. When subjected to the static magnetic field, MRI active nuclei, e.g., hydrogen nuclei, precess at frequencies proportional to the strength of the magnetic field. MRI signals are obtained from the precessing nuclei. Various properties of the MRI signals depend upon the precessional frequency and, consequently, the magnetic field experienced by the nuclei.
In general, the static magnetic field is inhomogeneous so that the field strength varies as a function of location within the object. Thus, the precessional frequency of the nuclei also varies within the object. In many situations, higher quality magnetic resonance images may be obtained using more homogenous magnetic fields in which field strength variations are reduced or eliminated. Accordingly, MR imagers generally include shim magnets that generate shim fields used to reduce variations in the static magnetic field. The process of adjusting the shim fields generated by the shim magnets is known as shimming. Shim magnets are typically electromagnets so that the strength of the shim fields can be controlled electronically.
Shimming typically requires a determination of variations in the static magnetic field to determine the proper shim field strengths. Variations in the magnetic field may be determined using a frequency map, which is indicative of the precessional frequencies of MRI active nuclei within each pixel of a slice, i.e., at different locations within the object. The spatial variation of the static magnetic field at different pixels is often presented as a field map.
Certain objects can be challenging to shim. For example, objects that move, e.g., the heart, thorax, and abdomen, may change position within the field of the imager and are thus difficult to shim, because they may not return to exactly the same position between movements, e.g., between heartbeats or inhalations.